1. Field of the Invention
This invention relates to aromatic polyamide having excellent thermal resistance, and particularly relates to polyamide which retains elastic modulus even at high temperatures above the glass transition temperature of the same.
2. Description of the Prior Art
Polyamide is widely used in various fields such as industrial materials, protective clothings and interiors due to its excellent mechanical properties, thermal resistance, flame resistance, electrical properties and chemical stability. Aromatic polyamide which has been industrially manufactured and marketed to date is divided into two main classes. One is represented by so-called para-base aromatic polyamide such as poly(para-phenyleneterephthalamide) (hereinafter abbreviated as PPTA). It has characteristics of high crystallinity, high melting point, high mechanical strength, low elongation and high elastic modulus whereas has poor processing ability as a result of stiffness and low elongation. Since para-base aromatic polyamide is also slightly soluble in usual polar organic solvents such as dimethylformamide, dimethylacetamide and N-methylpyrrolidone, difficulty is encountered in film forming, spinning and other miscellaneous shaping processing. Therefore parabase aromatic polyamide is restricted in its application and only used as reinforcing filaments for tire cords, structural materials and composites.
The other is represented by so-called meta-base aromatic polyamide such as poly(metaphenyleneisophthalamide) (hereinafter abbreviated as PMIA). The meta-base polyamide has to some extent lower mechanical properties than para-base polyamide whereas has an excellent processing ability and is also soluble in the above polar organic solvents. Therefore meta-base polyamide can be applied for spinning and film casting, and is widely used for various fields such as flame retardant protective clothings and flameproof clothes, various industrial belts, heat resistant felt, and heat resistant insulation papers and adhesives.
PMIA, however, has lower crystallinity than PPTA and causes abrupt reduction in physical properties above the glass transition temperature Tg (above 220.degree. C.). Accordingly, PMIA exhibits excellent properties at temperatures below Tg whereas causes particular problems at high temperatures above Tg. Even instantaneous use or application over a short period of time leads to shrinkage or a steep reduction in properties, thereby field of use has been restricted. As a countermeasure for the prevention of shrinkage, for example, a heat treatment method of PMIA fibers at high temperatures has been disclosed in Japanese Patent Publication No. 17692/1978. The method is unfavorable because deterioration on the surface of fibers and reduction in properties are resulted from the treatment. Besides, as a method of heat shrinkage prevention and chemical resistance enhancement for. films, film forming methods from a polymer solution containing a crosslinking agent has been disclosed in Japanese Patent Publication No. 110128/1980 and 13742/1980. However selection of the crosslinking agent and optimization of film forming conditions are difficult in these methods.